The invention relates to a method and a system for linking SIP terminals, or terminals that are able to implement a call setup protocol, for example the SIP protocol (for Session Initiation Protocol), to residential gateways.
From the outset, it will be noted that the expression “residential gateway” is understood to refer to a client gateway, that is to say a gateway situated user side, in contradistinction to an item of network equipment situated in the network under the control of the operator. It may, by way of nonlimiting examples, be a gateway for domestic use within a residence or a gateway for professional use within a company. In a general way, it is a gateway for client use.
Numerous offers of telephone facilities over the IP network are, at the present time, based on domestic installations composed of a residential gateway linking one or more mono or multimedia terminals, such as visiophone analog telephones, personal computers, digital televisions for example.
The aforesaid residential gateways are also referred to as intelligent residential gateways, since they make it possible, in fact, to link various categories of networks.
The present invention is aimed more particularly at the functionalities of residential gateways intended for real time electronic communication services, such as VoIP telephony (voice over IP), video-telephony for example.
The aforesaid residential gateways communicate with the operator's network by virtue of a call control protocol whose objective is the creation of a VoIP type communication context between two IP terminations. The objective of the aforesaid SIP protocol defined by the IETF ({tilde under (I)}nternet {tilde under (E)}ngineering {tilde under (T)}ask {tilde under (F)}orce) is to allow setup, modification and termination of multimedia sessions in an IP network or within the framework of architectures for controlling networks based on IP transport.
The role of residential gateways in exchanges of SIP signaling depends on the type of terminal linked up:                when the terminal linked up is not an IP terminal such as an analog, RNIS, Bluetooth, DECT or other terminal, the gateway plays the role of SIP termination by performing the translation between the SIP signaling exchanged with the network and the signaling exchanged with the terminal. This situation corresponds to the case of the “VEG” VoIP Embedded Gateway function represented in FIG. 1a, this type of terminal being referred to as a non-IP terminal in the present patent application;        when the terminal linked up is of IP type, any SIP terminal, the residential gateway behaves as a simple router and steers the SIP messages to the terminal or terminals concerned. The latter then play the role of termination for the SIP protocol.        
Current residential gateways may operate according to two operative modes making it possible to manage multiline connection, such as represented in FIG. 1a. 
The SIP protocol comprises a registration phase, termed the REGISTER phase, the object of which is the creation, in the network, of an association between the public identity of a user, on the basis of which the latter can be contacted, telephone number or SIP Universal Resource Identifier URI, and the network address or addresses of the terminals enabling the network to contact them. The aforesaid association is stored in a location database.
The first operative mode represented in FIG. 1b, consists in declaring each terminal, separately in relation to the network, situated or not situated behind the same residential gateway, even when these terminals share one and the same public identity. The network then acts as if the declared terminals were not hosted by the same residential gateway.
In this multiline operative mode during the registration phase, each declared terminal announces its network address, IP address+port pair, and its public identity, telephone number. The corresponding association is stored in a database so as to be interrogated by a specific item of equipment, such as an SIP proxy, handling the trunking of the incoming calls to their final destination(s).
Each terminal has a priori a different network address from the others, independently of the fact that it does or does not share its public identity with another terminal.
Within the framework of the aforesaid operative mode, one and the same public identity thus points to several network addresses. With each incoming call to a given public number, on receipt of a call setup message, the SIP proxy forwards this message to each of the SIP destinations through interrogation of the database. In this automatic call distribution system referred to as “forking”, the residential gateway merely routes the incoming IP packets, as would a simple IP router.
A second operative mode, represented in FIG. 1c, consists in incorporating into the residential gateway an application package processing allowing the gateway to offer the network a single physical interface by handling the final trunking of the incoming calls to the appropriate SIP destination or destinations, that is to say the terminals.
During the registration phase, the residential gateway sends registration messages for the account of the terminals that it links.
Several modes of implementation are conceivable, depending on whether the terminals send or do not send any registration messages.
Under the first assumption, the residential gateway intercepts the “REGISTER” registration messages sent by the SIP terminals and modifies them so as to appear as the single SIP destination for the whole set of numbers attached to the terminals that it connects. Furthermore, the residential gateway stores in an internal database the correspondence between the public identity allocated to the terminal and the associated network address.
Under the second assumption, the residential gateway determines on the basis of configuration data the associations between network addresses of the terminals and corresponding allocated public identities, then sends one or more “REGISTER” registration messages on the basis of these associations, here again passing itself off as the single SIP destination to be contacted. On receipt by its internal SIP processing logic of an “INVITE” call setup message, the residential gateway determines the public identity targeted by the call, by inspecting the header of the SIP message “To”, then interrogates its internal database, so as to retrieve the SIP terminal or terminals concerned so as to forward the “INVITE” call setup message to them.
The first and second operative modes of the prior art exhibit the drawbacks hereinafter.
The solution according to the first operative mode according to the “forking” technique requires the storage at each instant by the network of the linkup topology information for all the residential gateways. In addition to the aforesaid storage, this operative mode leads to the generation of more signaling than is necessary. Specifically, the operations of registering/de-registering the terminals lead systematically to exchanges of registration messages. Moreover, during an incoming call, a call setup request must be sent to each of the terminals. The network must then take on board the cancellation of the transactions with no response by the targeted terminal.
The solution according to the second operative mode, palliative processing incorporated into the gateway, exhibits the drawback of adding non-trivial application procedures to the residential gateway, as well as an internal mini-database. This solution therefore appears to be prejudicial to the performance of the residential gateway.
The subject of the present invention is to remedy the drawbacks of the aforesaid prior art solutions.